An undesirable electrical instability due to positive sodium ions often results in silicon semiconductor devices subjected to an oxidizing process. It would be desirable to remove, or getter, such ions in a simple processing step without adversely affecting any characteristics of the device.
The prior art teachings of sodium gettering include U.S. Pat. No. 3,529,347 issued to R. Ingless et al. on Sept. 22, 1970. The patent discloses applying a thin layer of phosphorous glass over an oxide, removing the layer after a short period of high temperature sodium gettering, and then keeping the temperature of the oxide low enough to avoid recontamination with sodium during subsequent processing. It would be desirable to eliminate the step of removing the gettering agent, and it would also be desirable to getter after subsequent processing steps without again introducing a gettering agent. Accordingly, it would be desirable to use a gettering agent at a concentration sufficiently low so as not to affect the chemical and mechanical properties of silicon dioxide, in order that the gettering agent should not require removal. Moreover the technique of gas-gettering with hydrochloric acid (HC1) vapor suffers from HC1 corrosion of the equipment.
Fabricating semiconductor devices includes such techniques as creating regions of stored charge. For example, stored positive charge has been used in the fabrication of a variety of metal-oxide-semiconductor (MOS) devices. In particular, a stable negative charge distribution would be desirable in certain n-channel MOS devices, such as n-channel transistors; but the production of such a charge distribution has presented difficulties.
The prior art teachings of storing charge in an oxide layer include U.S. Pat. No. 3,796,932, issued to G. F. Amelio et al. on Mar. 12, 1974, and U.S. Pat. No. 3,877,054, issued to D. M. Boulin et al. on Apr. 8, 1975. Both of these patents teach charge distributed within an insulator disposed on a silicon substrate.
The aforementioned G. F. Amelio patent suggests storing negative charge in an oxide layer by using p-type dopants such as boron. However, such dopants appear not to store a negative charge of sufficient stability for as long as is desired for certain devices. The aforementioned Boulin et al. patent teaches forming an electrically alterable memory by introducing charge trapping materials such as tantalum and restricting them to an interface between two different insulation layers disposed on a semiconductor substrate. The layer of material creates sites which can be filled with electronic charge carriers and subsequently emptied under the influence of an electric field applied across the structure. In contrast, there are devices in which it is desired to have negative charge storage which is not electrically alterable and which forms a permanent part of the device.
Fast surface states are known in the prior art and have been considered undesirable in many semiconductor devices. We believe that currently there are no commercial devices dependent on fast surface states because of the difficulty to produce and control them at will. It would be desirable to simplify control and production of fast surface states so that they may then be advantageously employed in semiconductor devices.
It is well known that the orientation of silicon and the mode of oxidation affects the number of fast surface states. However, such fast surface states are temperature sensitive and are affected by annealing. An article entitled "Determination of Deep Energy Levels in Si by MOS Techniques" by W. Fahrner and A. Goetzberger appearing in Applied Physics Letters, Vol. 21, No. 7, Oct. 1, 1972, page 329, teaches implanting tantalum into a silicon-silicon dioxide interface and producing fast surface states. However, the article does not teach forming nonannealable fast surface states at full thermal equilibrium. It would be desirable to controllably increase the number of nonannealable fast surface states.
In addition to improving sodium gattering, storing stable negative charge and creating nonannealable fast surface states, it would be desirable to attain these improvements with few and simple processing steps. Further, it would be desirable to develop devices which can advantageously use these improvements.